The present invention relates to a rotary particle-drying drum. More particularly this invention concerns such a drum used to dry wood chips for the production of oriented-strand board.
Commonly owned U.S. Pat. No. 6,119,363 described a rotary drying drum comprising a generally cylindrical outer wall centered on and rotatable about a horizontal axis and provided with axially rowed sets of axially extending baffles that extend radially inward from outer edges attached to the outer drum wall. These baffles are of zig-zag shape so as to form pockets and have inner ends that stop somewhat short of a central shaft extending through and carrying the drum. Each baffle has several sections extending at an angle of about 90xc2x0 to one another and forming at least two angularly open pockets and the sets of baffles are set so the material cascades from the pockets of one set of baffles to the next as the drum rotates. Around twenty such baffles in each set extend to close to the center shaft at the axis of the drum. Struts extending from an inner ring stabilize the baffles.
In order to dry wood chips between 5 mm and 50 mm wide and 75 mm to 175 mm long the drum has fewer, normally about twelve, baffles that have inner ends spaced somewhat farther out from the drum axis. No inner tube is used to dry these larger chips.
As the drum diameter is increased to increase the capacity of the dryer, it becomes necessary to similarly increase the number of baffles. As a result their spacing at their radial inner ends can become quite small so it has been suggested to shorten some of the baffles in order to prevent clogging. Such shortening reduces the contact area and, thus, the efficiency of the dryer. Drums of large diameter, 5 m to 6 m, cannot have a properly dimensioned inner tube.
German patent 3,345,118 of Otto Brudern describes a sugar-drying rotary drum having axially succeeding sets of outer axially extending baffles each having an outer edge secured to the drum, extending radially inward therefrom, and each forming an angularly open pocket. Axially succeeding sets of inner axially extending baffles each have an inner edge secured to a center shaft, extend radially outward therefrom, and each form an angularly open pocket directed angularly opposite to the outer pockets. Thus sugar fed in one end of the drum is poured from the outer pockets to the inner pockets across a considerable space between the inner edges of the outer baffles and the outer edges of the inner baffles.
Wood chips do not flow like sugar particles so such a rotary drum is not efficient at drying these types of particles. Dropping the particles through a substantial radial gap results in clumping and jamming. Furthermore the rigid interconnection of parts in this and similar systems causes considerable stresses when they are heated and expand. Thus the baffles can become loose as the fatigued joints where they are connected to the drum and shaft fail. Furthermore the known drums are not often sufficiently stiff with respect to torque so that, especially when heated, they can deform.
It is therefore an object of the present invention to provide an improved rotary particle-drying drum.
Another object is the provision of such an improved rotary particle-drying drum which overcomes the above-given disadvantages, that is which is particularly effective at drying wood chips and that at the same time is of durable construction that can be counted on to have a long service life.
A rotary particle dryer has according to the invention a shaft extending along an axis and a hollow drum coaxially and spacedly surrounding the shaft and rotatable about the axis in a direction. A set of axially extending inner baffles fixed to the shaft extend radially outward therefrom and each form at least one angularly open inner pocket. A set of axially extending outer baffles fixed to the drum extend radially inward therefrom and each form at least two angularly open outer pockets. In accordance with the invention structure mechanically connects the inner baffles directly with the outer baffles.
Thus the generally zig-zag shaped baffles are limitedly elastically deformable radially of the axis and can readily deform when heated without damage to the structure. The baffles are cascaded to make this even more effective, and also ensures that the material being dried flows from the outer baffles to the inner baffles for best possible drying. On the other hand the baffles have a certain torsional rigidity so that the drum will not go far enough out of shape to be damaged. Securing the outer baffles to the inner baffles rather than to other structure in the drum reduces the likelihood of blocking the flow of particles, here normally chips for oriented-strand board, through the machine.
The structure according to the invention includes an inner ring fixed to the inner baffles, an outer ring fixed to the outer baffles, and a plurality of struts extending radially between and fixed to the rings. Both of the rings are centered on the axis and the struts extend generally radially. Normally the struts and rings are made of sheet steel.
At least some of the outer baffles have rear faces directed backward in the direction and provided with stiffening reinforcement. This reinforcement is respective radially and angularly extending bars fixed to the rear faces.
Furthermore according to the invention the drum has a predetermined inner radius from the axis, and the inner baffles have a radial dimension equal to between 20% and 40% of the radius. Similarly the inner baffles have radial outer ends spaced from inner ends of the outer baffles by a radial distance equal to between 5% and 15% of the radius. Such dimensions are particularly effective with chips between 100 mm and 150 mm long and between 20 mm and 50 mm wide in a drum having a diameter of 5 m to 6 m.
According to the invention there is an even number of the outer baffles and there are half as many inner baffles as outer baffles. The outer baffles include long outer baffles and short outer baffles alternating with the long outer baffles. The inner baffles are radially generally aligned with the short outer baffles.